With a control system of a wound induction machine, in the event of a breakdown, such as a short-circuit, of an electric power system, a circuit breaker connected to the stator side (primary side) of the wound induction machine separates the wound induction machine from the electric power system. At this time on the stator side, the abnormal current phenomenon (so-called Current zero-missing phenomenon) in which there is no timing of when the alternating current (primary current) becomes zero may occur. In this case, an inverter connected to the rotor side of the wound induction machine (secondary side) switches to an irregular operation mode.
Accordingly, a direct current component is superimposed on an alternating current (secondary current) output from the inverter. Consequently, a timing of when the alternating current becomes zero (so-called a zero cross point) is generated on the stator side of the wound induction machine.
Besides, in the event of a breakdown of the electric power system, breakdown current may flow into the wound induction machine. In this case, a switching element used as the inverter is immediately turned off. However, if the off period of the switching element is long, the direct current voltage input to the inverter increases, which may damage the switching element. To avoid it, a chopper is operated to suppress an increase in the voltage.
If, before a breakdown occurs in the electric power system, the wound induction machine operates in such a manner that high reactive power, that is, strong excitation occurs in the leading direction when viewed from the electric power system, high breakdown current occurs upon the breakdown of the electric power system. In this case, the off period of the switching element is extended, and the direct current voltage easily increases. For this reason, to suppress such an increase in the direct current voltage, the chopper is required to be a resistor with high power consumption. If the power consumption of this resistor is high, the size and cost of the device may increase.